1. Field of the Invention
The present invention relates to a centrifugal liquid separation machine, and in particular to a screw type centrifugal liquid separation machine having a continuous decanter and using pressurized air to promote solids transport.
2. Description of the Related Art
Centrifugal machines are useful in many types of applications. In one application, wastewater treatment plants, it is desired to achieve a 4% to 6% cake solids discharge. This range of cake solids is required in order for an anaerobic digester to operate efficiently. Falling below this range requires increased digester capacity. Rising above this range typically results in mixing problems due to the thickness of the heavy phase liquids. Even though the principles of the present invention are described with respect to one type of application, it is understood that the invention is in no way limited to this described application.
In the basic form, decanter type centrifugal separation machines have a rotating outer bowl, an internal screw conveyor co-axially aligned with the outer bowl, and a mechanism for maintaining a difference in speed between the rotating outer bowl and the internal screw conveyor to allow for continuous operation of the machine. Rotation of the bowl at elevated speeds results in solid liquid separation action within the separation region of the machine due to elevated levels of gravitational forces within the machine. Materials such as solids and heavier density liquid will thus settle to the outer diameter of the separation region and the lower density liquid will migrate to the inner diameter of the separation region. The separation rate increases with the elevation of gravitational forces resulting from the rotation of the bowl. The screw conveyor has a rotational speed greater or less than the rotational speed of the outer bowl. This difference in speed allows screw conveyor flights to provide a mechanical sweeping action within the separation region.
There have been many centrifuge designs over the years to deal with the challenges of soft more difficult to convey solids. A few of those designs are illustrated in the following US patents.
U.S. Pat. No. 3,795,361 to Lee is titled Centrifuge Apparatus. This patent describes how a decanter centrifuge having a screw conveyor within an imperforate bowl is provided with an annular baffle carried by the screw conveyor. A heavy phase discharge port is taught to be located in a tapered portion of the bowl and is located at a greater radial distance from the rotational axis than the inner surface of the light phase material. The periphery of the baffle is closely spaced from the bowl in order to form a restricted passageway for the underflow of heavy phase material from a separating zone within the cylindrical portion of the bowl to a heavy phase discharge zone within the tapered portion of the bowl. With a conical baffle, incoming feed is directed onto the inwardly facing surface of the baffle and accelerated in order to minimize turbulence in the separating zone. The use of a tapered portion, or a beach, reduces the capacity of the machine, as shallow beach angles required to adequately convey grit or trash requires an undesirably large proportion of bowl length.
U.S. Pat. No. 4,339,072 to Hiller is titled Centrifuge for Separating Solids/Liquids Mixtures. In this invention, a centrifuge drum having an outer jacket is provided with apertures positioned in the jacket. Through the apertures at least a partial discharge of concentrated solids phase occurs thereto. A control device preferably in the form of a disk provides a surface spaced at a small interval from the apertures so as to prevent the flow of solids/liquids through the aperture except when a discontinuity such as a recess or cut-out in the surface occurs so as to allow flow through the aperture. While this patent describes a solution for eliminating a truncated cone by discharging from the outer bowl, its design is not without drawbacks. For example, it is required that all solids pass through very small nozzles. This can result in undesirable amounts of abrasive damage and plugging of the machine.
U.S. Pat. No. 5,542,903 to Nishida et al. is titled Centrifugal Liquid Separating Machine Using Deceleration Vanes. This patent teaches that discharge passages for concentrated and separated liquids are separately formed in shafts of a rotary bowl and a screw conveyor. In an inlet passage of the radial discharge passage leading from the inside of the rotary bowl to the discharge passage in the shaft, an annular space is divided into sectors by a plurality of deceleration vanes which are mounted on the screw conveyor and extend in a radial direction from the axis of the machine. While this patent shows a solution to problems with amorphous trash, is does not address the problems caused by abrasive materials such as grit.
U.S. Pat. No. 4,898,571 to Epper et al. is titled Solid Bowl Centrifuge. This patent illustrates a method and apparatus for separating mixtures of different densities into a lighter phase and a heavier phase including a rotary truncated cone shaped drum providing a cylindrical settling sump at the outer wall, a displacement member rotatably located within the drum forming a settling sump between the displacement member and the drum wall, a discharge element for lighter phase material spaced radially inwardly from the settling sump, a discharge conductor for heavier phase material leading from the settling sump at the deepest location at the outer circumference of the drum, and a compressed air conduit connected to the discharge for heavier phase material aiding in the removal thereof, and vanes on the displacement member aiding in movement of the material through the drum. The lighter and heavier phase materials both exit the apparatus at the same end of the apparatus. Yet, both the bowl and back drive are shown to be on the save end as the feed introduction point, and conventional back drive systems of that era did not allow for the center axis flow of process materials.
U.S. Pat. No. 5,176,616 to Schlip et al. is titled Method and Apparatus for the After-Treatment of the Thick Material Discharge Region of a Solid Bowl Worm Centrifuge. This patent teaches the use of a centrifugal solid bowl worm separator having an outer cylindrical drum and an inner rotatable worm with helical flights on the outer surface with the drum arranged to receive a material to be separated in light and heavy fractions or phases and the mechanism provides for a method of injecting and mixing a fluidizing substance into the heavy fraction material within the drum before it leaves the drum to mix with the heavy fraction within the drum. The fluidizing substance lowers the density of the heavy fraction to enable its discharge by the worm flights. The hydraulic pressure of the liquid helps push the heavy fraction out of the separator which prevents a reverse flow and intermixing of the heavy fraction with the liquid fraction. However, it is apparent that this patent does not illustrate a suitable structure for adequately mixing air with sludge, and therefore the design can be improved upon.
U.S. Pat. No. 5,244,451 to Retter is titled Method for Operating a Worm Centrifuge Having a Pressurized Gas Introduction. This patent shows a method for operating and improving the throughput and efficiency of a worm centrifuge by introducing, at a controlled frequency, successive pressure surges into the concentrated sludge fraction within the bowl separator preceding the solids discharge opening whereby the pulse frequency and the level of pressure are controllable and can be controlled as a function of the sludge fraction throughput through the separator. This patent shows the use of a pulsating airflow as a means to overcome air distribution short circuiting in the cake discharge path. In this regard, it does not show a continuous induction of air.
U.S. Pat. No. 4,790,806 to High is titled Decanter Centrifuge Incorporating Airlift Device. This patent shows a decanter centrifuge which includes an annular bowl, a hollow tube on the axis of the bowl, and means for discharging from the bowl a first phase of an input sludge, the centrifuge being characterized by a fluid-activated airlift device which includes a discharge line radially supported from the hollow tube, and a fluid supply line for conveying fluid from within the hollow tube to an outer end portion of the discharge line to effect removal from the bowl through said line of another phase of the sludge. The air in this invention is taught to be conducted by pipe line through the hub. Coarse particles of the heavy phase material are prevented from entering the airlift device by virtue of a narrow clearance between the sludge inlet to the airlift device and the inside surface of the wall of the bowl. If oversized particles are removed from (or absent in) the feed slurry all of the sedimented solids can be discharged by means of the airlift device, and the conical-beach portion of the decanter bowl is not required. Implicit in this teaching is the limitation that the beach is required when oversized particles are not removed from the feed slurry. Also problematic is that success of localizing the article transport was short circuiting the hydrodynamic effects in the cylindrical portions of the unit.
None of these patents show a design with an air delivery system entering through a back drive system.
None of these patents show a design with continuous axial air inlet behind a solids baffle in a screw type centrifugal system.
None of these patents show a design with uniform spacing of fine air particle inlets across a radial cross section of the flow pathway.
None of these patents show a design with turbulence induction to at least partially re-suspend grit in discharge flow path.
None of these patents show a continuous process control operated by measuring the properties of the heavy phase discharge stream with a sensor, and accordingly adjusting the continuous air supplied to achieve a desired heavy phase discharge consistency.
Thus, there exists a need for a centrifugal liquid separation machine that solves these and other problems.